1. Field of the Invention
The invention relates to the field of propulsion systems for aircraft and, in particular, to propulsion systems for vertical and/or short take-off aircraft (V/STOL).
2. Description of Related Art
The efficiency of a propulsion system for an aircraft is maximized when the velocity of the exhaust gases equals the velocity of the aircraft in its flight direction at minimum specific fuel consumption. Thus during take off, landing and hovering, it is obvious that a helicopter, which provides a small incremental velocity to a large mass of air (low disc loading), is more efficient than a jet aircraft, which provides a large incremental velocity to a small mass of air (high disc loading). However, a helicopter, because of its very large diameter rotor, has a limited forward velocity, of less than 200 Knots due to compressibility effects on the rotor blade tips. Thus most V/STOL aircraft are compromises, which either limits the forward velocity of the aircraft (helicopter) or requires oversized engines for vertical flight (jet aircraft) causing a loss in cruise efficiency.
For example, the AV-8A Harrier V/STOL aircraft utilizes a turbofan engine for both hover and cruise propulsion. The turbofan engine was sized to produce adequate thrust for vertical lift in hover, but its correspondingly large frontal area increases the drag of the aircraft and limits its maximum speed to less than Mach 1 (approximately 580 Knots at sea level). However, the turbofan exhaust is of significantly high velocity and, thus propulsion efficiency is low at cruise velocities because the engine is oversized for this flight mode and hovering, which requires maximum power, for any significant amount of time is avoided because of the high fuel consumption.
In U.S. Pat. No. 4,474,345, "Tandem Fan Series Flow V/STOL Propulsion System," by R. G. Musgrove, a jet engine with a small fan, which is capable of providing supersonic performance, is modified to provide vertical lift. The basic engine fan is split to provide fore and aft fans connected by means of a common drive shaft. The fans are centrally mounted in a duct located within the aircraft along its longitudinal axis. In normal wingborne flight, (hereinafter referred to as horizontal flight mode), the fans operate in series with the fan exhaust mixing with the turbine exhaust and exiting through a nozzle located at the rear of the aircraft. In the vertical mode of operation, a diverter is positioned downstream of the forward fan and is movable to a position for diverting the exhaust from the forward fan downward relative to the longitudinal axis of the aircraft, while simultaneously opening an auxiliary inlet for permitting the introduction of air to the aft fan. An aft diverter is located in the nozzle which is also moveable to a position for diverting the exhaust from the aft fan and engine core downward. Thus for vertical flight the diverters are actuated causing the exhaust from both fans and the core engine to be directed downward fore and aft of the center of gravity of the aircraft. However, the tandem fan engine has less thrust in the vertical takeoff and landing mode of operation than it has in the normal flight mode of operation. The thrust is greater in cruise because airflow passes through both fans, and thus the core is supplied with air that is raised to a higher pressure level (supercharged); whereas, in the vertical mode the core engine airflow passes through only the aft fan. Consequently, the tandem fan concept is not an efficient design for a V/STOL aircraft.
Another more efficient approach is to couple a separate large diameter lift fan to the main turbofan by means of a drive shaft. The lift fan is clutched in and powered only during vertical flight modes. In addition, both the fan section and turbine section exhaust are deflected downward to provide lift. Increased performance is obtained because some of the turbofan's power is being used to drive the lift fan, which is more efficient at the low vertical take-off and landing speeds. Such a system can be found in co-pending U.S. patent application Ser. No. 07/521,211, "Propulsion System For A V/STOL Aircraft," filed May 5, 1990. However, as with all the designs discussed above, the propulsion systems are designed primarily for supersonic high-speed flight and modified for V/STOL operation. They are not readily applicable for subsonic aircraft where significant hover time is required.
In U.S Pat. No. 4,791,783, "Convertible Aircraft Engine," by R. E. Neitzel, a turbofan concept is disclosed for converting almost all the power used by the engine fan to shaft horsepower to drive a helicopter rotor. Guide vanes located on both sides of the outer portion of the engine fan can be actuated to block off air flow through the fan duct while still allowing air flow into the engine core. A gear mounted on the forward end of the fan shaft is coupled to a drive shaft which in turn drives the rotor. Such a system provides maximum efficiency during take off and landing and also during normal flight. However, if high-speed flight (greater than 0.5 Mach), is to be accomplished, the rotor must be either stopped (x-wing concept) or stopped and stowed. The former concept requires an extremely complex computer-controlled pneumatic blowing system that, to this date, has not been successfully developed. The latter system causes a severe weight penalty and requires a complex folding and stowing system. Furthermore, it is difficult to achieve low-observable (LO) characteristics with either design.
The tilt rotor concept found in the V-22 Osprey aircraft, uses large diameter propellers powered by two cross-shafted turboshaft engines. Its disc loading is higher than a helicopter, but lower than a turbofan and, thus is efficient in the vertical flight modes; however, the large propellers limit the top speed to about 300 Knots at sea level. Again, this is due to compressibility effects on the propeller tips. Furthermore, the large propellers eliminate it as a candidate for missions where a low radar cross-section is required. Tilt pylon-mounted turbofan engines can obtain a higher cruising speed, but lose vertical flight mode efficiency because of the high-disc loadings. In addition, pylon-mounted engines of any type, where the fan is visible to radar signals, are also unsuitable for LO missions.
The type of V/STOL aircraft that appears to be most suitable for missions where low radar cross-section is required is one where the entire propulsion system is imbedded in the aircraft wing and/or fuselage. For example, as in a ducted fan-in-wing for the vertical flight mode and turbojet or turbofan engines for the horizontal flight modes. The overall concept is rather old, dating at least back to 1914. For example, U.S. Pat. No. 1,130,623, "Flying Machine," by M. L. Mustionen, discloses pylon-mounted lift propellers and a pusher propeller mounted in the tail, all powered by a single-piston engine. However, with modern V/STOL aircraft, safety requirements dictate the use of multiple engines with cross-shafting to obtain engine-out performance in the vertical flight mode. Examples of this concept can be found in U.S. Pat. No.'s 4,828,203, "Vertical/Short Take-Off And Landing Aircraft," and U.S. Pat. No. 4,469,294, "V/STOL Aircraft," both by R. T. Clifton, et al. This aircraft design uses two pylon-mounted ducted propellers for the vertical flight mode and a rear-mounted ducted propeller for the horizontal flight mode. Two engines are mounted in the airframe and "belt drive" a common shaft that is directly connected to the rear-mounted propeller. The drive shaft is also connected to a right-angle gearbox which in turn drives the two pylon-mounted ducted lift propellers by means of belt drives. It is apparent that such a combination aircraft design and propulsion system, as configured, does not lend itself to LO missions because of the rear-mounted ducted propeller used for the horizontal flight mode. However, even if it were installed in a proper airframe, it still would not provide the necessary propulsion efficiency and engine-out performance required for any practical aircraft.
The basic problem is that in an aircraft, such as a transport, the ratio of thrust required for takeoff in the vertical flight mode to that required for efficient cruise in the horizontal flight mode is on the order of 10 to 1. Having multiple engines simply to provide for engine-out capability yields a thrust mismatch between the cruise and vertical flight modes. If the aircraft has only two engines and it requires both engines for a normal takeoff in the vertical flight mode, then each engine alone must be able to provide the total thrust required (in a max power setting for engine-out capability. This means that each of the two engines must be greatly oversized and, therefore, will yield very poor cruise efficiency. It's either this approach or stay with a single engine, as in the AV-8A Harrier aircraft.
In applicants' co-pending patent application U.S. patent Ser. No. 07/917,241 "Propulsion System For An Aircraft Providing V/STOL Capability," filed Jul. 22, 1992, this problem is addressed. In detail, the invention includes a pair of ducted lift fans mounted on the aircraft for providing thrust in the vertical flight mode. A pair of ducted cruise fans is mounted in the aircraft for providing thrust in the horizontal flight mode. Two sets of turboshaft engines are mounted in the aircraft with each of the sets comprising a plurality of the turboshaft engines. Each turboshaft engine includes an output shaft and has an optimal power output sufficient for powering one of the pair of ducted cruise fans in the horizontal flight mode. Each of the sets includes a sufficient number of the turboshaft engines to provide an optimal power output for powering one of the pair of ducted lift fans and one of the pair of ducted cruise fans in the vertical flight mode. A shafting system is mounted on the aircraft for coupling all of the turboshaft engines to the first and second pairs of ducted fans. A first decoupling system is connected to the shafting system for decoupling the pair of ducted lift fans from the first and second sets of turboshaft engines. Finally, a second decoupling system is connected to the shafting system for individually decoupling each of the turboshaft engines from the shafting system. This invention incorporated various combinations of turboshaft engines and shafting systems to provide power for vertical lift, good cruise efficiency, and one-engine-out capability. However, the mechanical shaft connection between the combiner gearbox and ducted lift fans imposes weight and/or design integration problems. The minimum weight combiner gearbox is one in which the power output shafts are either 90 degrees or 180 degrees to the power input shafts. Since the ducted lift fans in the wing need to be located at the center of gravity, to minimize the lift fan pitching moments, the combiner gearbox is forced to be located near the center of gravity to keep the propulsion system light. This feature reduces the flexibility of the designer in locating the turboshaft engines and cruise fans along the longitudinal axis of the aircraft, since these systems need to be close to the combiner gearbox. In addition, good design practice dictates certain length inlets and exhaust ducts (especially in low observable applications) such that this restriction on the locating of the cruise fans, combiner gearbox and turboshaft engines results in very limited design flexibility with respect to configuring the fuselage.
To eliminate these potential weight and integration problems, hot gas driven ducted lift fans have been substituted for the shaft-driven ducted lift fans. An example of this approach can be found in U.S. Pat. No. 3,972,490, "Trifan Powered V/STOL," by V. H. Zimmermann. In this invention, the exhaust from two turbine engines is connected by ducts to a nose-mounted, tip-driven, ducted lift fan and two pylon-mounted ducted cruise fans. The cruise fans, which are also tip driven, incorporate fan exhaust deflectors that are extended to divert the fan exhaust downward during the vertical flight mode. During the horizontal flight mode, the ducts to the ducted nose-mounted lift fan are closed off by means of valves. In U.S. Pat. No. 3,033,492, "Cruise Lift Fan System," by B. H. Rowe, a propulsion system is disclosed wherein two tip-driven ducted fans are mounted on pylons such that they can be rotated from the horizontal, for cruise, to the vertical for takeoff and landing. The two ducted fans are driven by exhaust gases from two turbojet engines or the like. This concept suffers from a requirement for large internal wing volume to accommodate the hot gas ducts between the turbine engines and the hot gas driven ducted lift fans.
While these inventions eliminate the weight of the gearboxes, shafts, etc., and the necessity of locating the combiner gearbox ams turboshaft engines near the center of gravity, it does not address the previously mentioned problem of matching the propulsion system to the ducted lift fans and ducted cruise fans when in separate flight modes and still provide engine-out capability. To date, no prior design has sufficiently addressed this problem in exhaust gas driven ducted lift fans.
Thus it is a primary object of the subject invention to provide a propulsion system for a vertical and/or short take-off and landing aircraft.
It is another primary object of the subject invention to provide a propulsion system for a vertical and/or short take-off and landing aircraft that provides increased propulsive efficiency.
It is a further object of the subject invention to provide a propulsion system for use in low-observable vertical and/or short take-off and landing aircraft.
It is a still further object of the subject invention to provide a propulsion system for a vertical and/or short take-off and landing aircraft that provides engine-out capability.
It is still further object of the subject invention to provide a propulsion system for a vertical and/or short take-off and landing aircraft that provides optimum or near optimum efficiency of the propulsion system in both the vertical and horizontal flight modes.
It is another object of the subject invention to provide a propulsion system for a vertical and/or short take-off and landing aircraft that provides increased design flexibility in locating the vertical lift fans relative to the other components of the propulsion system.